The mammalian thalamus is uniquely positioned to regulate diverse functions of the cerebral cortex including sensation, motor control, learning and memory and emotion. These functions critically depend on the proper development of dozens of thalamic nuclei, each of which exhibits characteristic patterns of afferent and efferent connections to and from different areas and layers of the cerebral cortex. Understanding how thalamic neurons are generated from neural stem cells and assembled to form specific nuclei during early development is needed to determine the underlying principles of thalamic organization. Despite recent advances in the study of developmental mechanisms responsible for laminar cytoarchitecture such as that of the neocortex, little is known about the formation of nuclear organization that is representative of many brain structures. Clonal lineage analysis has shown that in the neocortex, cohorts of neurons generated from individual neural stem cells form a radial column that spans the entire cortical wall and acts as a functional unit. In contrast we do not know whether thalamic nuclei are each composed of progeny derived from distinct neural stem cell populations, or if there is a spatial or temporal organization that determines the fate of neural stem cell progeny, which is not aligned with the anatomical borders of individual nuclei. The goal of the proposed research is to combine the expertise of two investigators to explore this important question. The Song laboratory has long been a leader in investigating the cellular and molecular mechanisms underlying hippocampal adult neurogenesis and has extensively used genetic lineage tracing at the clonal level to determine the patterns of proliferation and differentiation of adult stem cells. The Nakagawa laboratory characterized the molecular diversity of progenitor cell domains of the embryonic thalamus and has made significant contributions to understanding the mechanisms of patterning and neurogenesis in the developing thalamus. Based on preliminary data using a genetically-based single cell lineage tracing technique that employs the MADM (mosaic analysis of double marker) method, we hypothesize that individual neural stem cells in the thalamus produce postmitotic neurons that are distributed in radial columns that span several thalamic nuclei and that the repertoire of the thalamic nuclei that are generated from individual stem cells is determined by the initial position of the originating cell. Completion of the proposed experiments will be a critical first step towar understanding fundamental mechanisms governing the development and organization of nuclear structures in the brain. This knowledge will have important implications for future investigations of the functional and structural consequences of dysregulation within thalamic progenitor domains in early neural development.